York YK Manual page 8

FORM 160.75-EG1 (519)
Introduction (Cont'd)
OFF-DESIGN PERFORMANCE
Since the vast majority of its operating hours are spent at off design conditions, a chiller
should be chosen not only to meet the full load design, but also for its ability to perform
efficiently at lower loads and lower tower water temperatures. It is not uncommon for chill-
ers with the same full load kW/ton to have an operating cost difference of over 10% due
to part load operation.
Part load information can be easily and accurately generated by use of the computer. This
information is important to an owner's operating budget. As a result, part load information
has now been standardized within the AHRI Certification Program in the form of an Inte-
grated Part Load Value (IPLV), and Non-Standard Part Load Value (NPLV).
The IPLV/NPLV formulas from AHRI Standard 550/590 much more closely track actual
chiller operations, and provide a more accurate indication of chiller performance than
the previous IPLV/APLV formula. A more detailed analysis must take into account actual
building load profiles, and local weather data. Part load performance data should be ob-
tained for each job using its own design criteria.
8 JOHNSON CONTROLS